Class 10 / Science
Class 10 Science Light Reflection and Refraction Notes
Notes on reflection, refraction, mirrors, lenses, image formation, sign convention, and lens formula.
Topic introduction
Light is a form of energy that helps us see objects. In Class 10, the chapter mainly explains how light behaves when it reflects from mirrors and refracts through lenses. A strong understanding of ray diagrams, sign convention, and formulas makes numerical questions much easier.
Key points
- Reflection is the bouncing back of light from a surface.
- Refraction is the bending of light when it passes from one medium to another.
- Concave mirrors and convex lenses can form real and inverted images.
- Use sign convention carefully in mirror and lens formula questions.
Definitions
Reflection: The bouncing back of light into the same medium after striking a surface.
Refraction: The change in direction of light when it passes obliquely from one transparent medium to another.
Principal focus: The point on the principal axis where rays parallel to the axis meet or appear to meet after reflection or refraction.
Focal length: The distance between the optical centre or pole and the principal focus.
Power of a lens: The reciprocal of focal length in metres, measured in dioptres.
Formulas
- Mirror formula: 1/f = 1/v + 1/u
- Lens formula: 1/f = 1/v - 1/u
- Magnification for mirrors: m = -v/u
- Magnification for lenses: m = v/u
- Power of lens: P = 1/f, where f is in metres
Examples
- A convex mirror is used as a rear-view mirror because it gives an erect, diminished image and covers a wider field of view.
- If a lens has focal length 20 cm, its focal length in metres is 0.20 m, so its power is 1/0.20 = 5 D.
- A concave mirror can form a real and inverted image when the object is placed beyond the focus.
Mirrors and lenses
Spherical mirrors include concave and convex mirrors.
Lenses include convex and concave lenses, each with different image formation patterns.
Formula-based questions
Mirror formula and lens formula require correct signs for object distance, image distance, and focal length.
Magnification describes the size and orientation of the image.
Common mistakes
- Using centimetres in power questions without converting focal length to metres.
- Forgetting the negative sign of object distance in the Cartesian sign convention.
- Mixing up mirror formula and lens formula.
- Drawing rays without arrow marks or without showing the principal axis.
- Writing only the formula in numerical answers and skipping substitution with units.
Quick practice
- State laws of reflection.
- Define refraction.
- What is focal length?
- Write the lens formula.
Related practice
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